1. Field of the Invention
The present invention relates to image forming apparatuses and methods for forming an image based on a digital image. In particular, the invention relates to an image forming apparatus and an image forming method for forming a high-quality image by controlling exposure energy density.
2. Description of the Background Art
Image forming apparatuses represented by copiers and printers are requested these days to be more excellent in the image quality and reproducibility. In order to meet this request, regarding a digital image forming apparatus of the electrophotography system, a method has been proposed according to which the diameter of a spot formed on a photoreceptor is optimized for a dot pitch determined by resolution.
For example, Japanese Patent Laying-Open No. 58-152269 discloses a technique of optimizing exposure energy with respect to a relational expression of dot pitch, diameter of exposure spot, and exposure energy density. Japanese Patent Laying-Open No. 7-61036 discloses a technique of not only optimizing a spot diameter for a dot pitch but additionally considering photoreceptor characteristics to obtain a high-quality image.
In addition to the technique of optimizing a spot diameter, a method is proposed for improving the quality of an image by image processing.
Japanese Patent Laying-Open No. 63-64763 discloses a technique of achieving reproducibility for an isolated dot by detecting isolated one-dot print data from print data and correcting one bit preceding or following the detected data as the one-dot print data so as to improve the image quality.
However, if these techniques are applied to an image forming apparatus with a high resolution such as the one which exceeds 600 dpi, it is difficult to achieve a superior reproducibility for images having different dot densities.
Specifically, a problem in the techniques disclosed respectively in Japanese Patent Laying-Open Nos. 58-152269 and 7-61036 is that, if the resolution is ensured of a line pattern with a relatively high dot density, for example, having lines on every second lines (hereinafter referred to as "periodic line pattern"), the diameter of a dot in an isolated one-dot pattern having a low dot density (hereinafter referred to as "isolated dot pattern") is smaller than a desired value or the dot itself is not formed in some cases.
On the contrary, if the dot diameter of the isolated dot pattern is set at a value appropriate for resolution, the line width of the periodic line pattern increases and a contrast corresponding to the resolution cannot be ensured.
Therefore, it is highly difficult for an image forming apparatus with a high resolution to obtain an excellent reproducibility for both of the periodic line pattern and the isolated dot pattern merely by adjusting the diameter of a spot formed on a photoreceptor, and accordingly to obtain an output image with a superior reproducibility.
According to the technique disclosed in Japanese Patent Laying-Open No. 63-64763, print data is changed (corrected) so that the isolated dot pattern itself can be reproduced. However, the reproduced image is not exactly the original image. Further, the reproduced image could have an unnatural portion depending on cases. Therefore, this technique does not provide a fundamental solution to the issue of ensuring a superior reproducibility for both of the periodic line pattern and the isolated dot pattern as described above.
It is noted that the issue of ensuring an excellent reproducibility for both of the periodic line pattern and the isolated dot pattern relates not only to the resolution but to a difference of sensitivity, i.e., photoconductivity of the photoreceptor. Considering this, it could be possible to achieve such an excellent reproducibility for both of the periodic line pattern and the isolated dot pattern by decreasing the film thickness of the photoreceptor, for example.
However, the decrease in the film thickness of the photoreceptor results in deterioration of dark decay and sensitivity characteristics due to deterioration by wear caused by repeated use. A problem then arises concerning durability. A further problem regarding gradation which is not negligible is that a photoreceptor having a small film thickness cannot provide a satisfactory gradation.